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| United States Patent |
5,701,035
|
|
Teraguchi
|
December 23, 1997
|
Electrode structure and method for fabricating the same
Abstract
The electrode structure of the invention includes a p-type Al.sub.x
Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1,
x+y.ltoreq.1) semiconductor layer and an electrode layer formed on the
semiconductor layer. In the electrode structure, the electrode layer
contains a mixture of a metal nitride and a metal hydride.
| Inventors:
|
Teraguchi; Nobuaki (Nara-ken, JP)
|
| Assignee:
|
Sharp Kabushiki Kaisha (Osaka, JP)
|
| Appl. No.:
|
504101 |
| Filed:
|
July 19, 1995 |
Foreign Application Priority Data
| Current U.S. Class: |
257/747; 257/744; 257/E21.172; 257/E29.144; 257/E33.063 |
| Intern'l Class: |
H01L 023/48; H01L 023/52; H01L 029/40 |
| Field of Search: |
257/747,76,743,744
|
References Cited
U.S. Patent Documents
| 5422500 | Jun., 1995 | Tomokawa et al. | 257/77.
|
| 5432808 | Jul., 1995 | Hatano et al. | 257/77.
|
| Foreign Patent Documents |
| 5-291621 | Nov., 1993 | JP.
| |
Other References
Nakamura et al. , "P-GaN/N-InGaN/N-GaN double-heterostructure
blue-light-emiiting diodes" Jpn. J. Appl. Phys. (1993) 32:L8-11.
Lin et al., "Low resistance ohmic contacts on wide band-gap GaN" Appl.
Phys. Lett. (1994) 64:1003-1005.
|
Primary Examiner: Saadat; Mahshid D.
Assistant Examiner: Clark; Jhihan B.
Attorney, Agent or Firm: Morrison & Foerster, LLP
Claims
What is claimed is:
1. An electrode structure comprising a p-type Al.sub.x Ga.sub.y
In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1)
semiconductor layer and an electrode layer formed on the semiconductor
layer,
wherein the electrode layer contains a mixture of a metal nitride
containing a first metal and a metal hydride containing a second metal,
wherein the first metal and the second metal are different.
2. An electrode structure according to claim 1, wherein the metal nitride
containing the first metal is selected from a group consisting of ScN,
TIN, VN, CrN, ZrN, NbN, LaN and TaN.
3. An electrode structure according to claim 1, wherein the metal hydride
containing the second metal is selected form a group consisting of
YH.sub.2, CeH.sub.2, PrH.sub.2, NDH.sub.2, SmH.sub.2, EuH.sub.2,
YbH.sub.2, HfH.sub.2, PdH, TmH, ErH, HoH, DyH, TbH and GdH.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrode structure for a p-type
Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor device, one of the Group
III-V compound semiconductor devices containing nitrides, and a method for
fabricating the same. More specifically, the present invention relates to
an electrode structure having an ideal ohmic contact showing an extremely
small contact resistance between a semiconductor layer and an electrode
layer and a method for fabricating the same.
2. Description of the Related Art
Generally, in fabricating an electrode structure for an Al.sub.x Ga.sub.y
In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1)
semiconductor device, nitrogen, one of the elements constituting the
semiconductor, is likely to dissociate from the surface of a semiconductor
layer in the electrode structure when the semiconductor layer is formed.
Therefore, it is difficult to produce crystals satisfying a desirable
stoichiometric ratio. When the dissociation of nitrogen forms the
vacancies inside the crystal structure of the semiconductor layer, the
conductivity type of the semiconductor layer turns into n-type. Therefore,
in fabricating an electrode structure for an Al.sub.x Ga.sub.y
In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1)
semiconductor device, it is difficult to form a p-type semiconductor
layer.
As a method for turning a semiconductor layer containing nitrogen into a
p-type semiconductor layer, it is well known to dope the semiconductor
layer with magnesium (Mg) as an acceptor impurity. However, a p-type
Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor layer formed by a
commonly-used metalorganic chemical vapor deposition (MOCVD) method
contains a large amount of hydrogen inside semiconductor crystals. A part
of the hydrogen atoms are bonded with the Mg atoms functioning as an
acceptor impurity, thereby preventing the Mg atoms from functioning as an
effective acceptor. In order to activate the Mg atoms as an acceptor
impurity, the semiconductor layer is subjected to an electron beam
irradiation process or an annealing process within a nitrogen environment,
thereby forming the p-type Al.sub.x Ga.sub.y In.sub.1-x-y N
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor
layer. However, the p-type semiconductor layer subjected to such a process
does not have a carrier density high enough to form an ideal ohmic contact
between a semiconductor layer and an electrode layer.
On the other hand, various electrode structures usable for Group III-V
compound semiconductor devices containing nitrides such as
blue-light-emitting diodes have been conventionally developed. In the
proposed electrode structures, various kinds of metals are used to form
the electrode layer. For example, in order to form the electrode layer for
a p-type Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor device, gold (Au) is most
commonly employed ("P-GaN/N-InGaN/N-GaN Double-Heterostructure
Blue-Light-Emitting Diodes", S. Nakamura et al., Jpn. J. Appl. Phys.
(1993) p. L8). Japanese Laid-Open Patent Publication No. 5-291621
discloses that nickel (Ni), platinum (Pt) and silver (Ag) may be used in
place of Au as the metals for forming the electrode layer.
However, in the case of using Au for the electrode layer, the contact
resistance between the electrode layer and the semiconductor layer is
large, and therefore an ideal ohmic contact cannot be obtained. In
addition, the adhesiveness between the electrode layer and the
semiconductor layer is inferior and the physical strength of the
semiconductor device becomes disadvantageously weak.
On the other hand, in the case of using Ni, Pt or Ag for the electrode
layer, the resulting adhesiveness is surely superior to that of Au, so
that a more ideal ohmic contact can be obtained as compared with the case
of using Au. However, in the case of using these metals, the following
problems are caused, for example. In a light-emitting diode using these
metals for the electrode layer, a differential resistance value at the
current value of 10 mA is large, i.e., several tens of .OMEGA.s. In other
words, such a light-emitting diode has a high operational voltage, judging
from the current-voltage characteristics thereof. In addition, since a
laser diode using these metals for the electrode layer has a small
electrode area, the contact resistance is increased as compared with a
light-emitting diode. As a result, the operational voltage of the laser
diode becomes larger than that of a light-emitting diode. This is why an
electrode structure having a sufficiently ideal ohmic contact cannot be
obtained even by the use of these metals.
In consideration of these problems, a conventional electrode structure
using these metals for the electrode layer cannot be regarded as an ideal
electrode structure for a p-type semiconductor device. Therefore, an
electrode structure for a p-type semiconductor device having an ideal
ohmic contact showing an extremely small contact resistance between a
semiconductor layer and an electrode layer is eagerly sought.
SUMMARY OF THE INVENTION
The electrode structure of the invention includes a p-type Al.sub.x
Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1,
x+y.ltoreq.1) semiconductor layer and an electrode layer formed on the
semiconductor layer. In the electrode structure, the electrode layer
contains a mixture of a metal nitride and a metal hydride.
In one embodiment, the metal nitride is selected from a group consisting of
ScN, TiN, VN, CrN, ZrN, NbN, LaN and TaN.
In another embodiment, the metal hydride is selected from a group
consisting of YH.sub.2, CeH.sub.2, PrH.sub.2, NdH.sub.2, SmH.sub.2,
EuH.sub.2, YbH.sub.2, HfH.sub.2, PdH, TmH, ErH, HoH, DyH, TbH and GdH.
According to another aspect of the invention, a method for fabricating an
electrode structure is provided. The method includes a step of forming an
electrode layer on a p-type Al.sub.x Ga.sub.y In.sub.1-x-y N
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor
layer. The electrode layer is formed by sequentially depositing a nitride
forming metal and a hydrogen absorbing metal on the semiconductor layer.
In one embodiment, the method for fabricating an electrode structure
further includes a step of heat-treating the semiconductor layer and the
electrode layer after the nitride forming metal and the hydrogen absorbing
metal are sequentially deposited on the semiconductor layer.
According to still another aspect of the invention, a method for
fabricating an electrode structure is provided. The method includes a step
of forming an electrode layer on a p-type Al.sub.x Ga.sub.y In.sub.1-x-y N
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor
layer. The electrode layer is formed by simultaneously depositing a
nitride forming metal and a hydrogen absorbing metal on the semiconductor
layer.
In one embodiment, the method for fabricating an electrode structure
further includes a step of heat-treating the semiconductor layer and the
electrode layer after the nitride forming metal and the hydrogen absorbing
metal are simultaneously deposited on the semiconductor layer.
According to still another aspect of the invention, a method for
fabricating an electrode structure is provided. The method includes a step
of forming an electrode layer on a p-type Al.sub.x Ga.sub.y In.sub.1-x-y N
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor
layer. The electrode layer is formed by depositing an intermetallic
compound containing a nitride forming metal and a hydrogen absorbing metal
on the semiconductor layer.
In one embodiment, the method for fabricating an electrode structure
further includes a step of heat-treating the semiconductor layer and the
electrode layer after the intermetallic compound is deposited on the
semiconductor layer.
Thus, the invention described herein makes possible the advantages of (1)
providing an electrode structure having an ideal ohmic contact showing an
extremely small contact resistance between a semiconductor layer and an
electrode layer; (2) providing an electrode structure including a
semiconductor layer having a carrier density high enough to realize such
an ideal ohmic contact; (3) providing an electrode structure in which
crystals having an appropriate stoichiometric ratio are formed at the
interface between the semiconductor layer and the electrode layer; (4)
providing an electrode structure including a p-conductivity type
semiconductor layer containing nitrogen atoms; (5) providing an electrode
structure having excellent physical and chemical characteristics; and (6)
providing a method for fabricating such an electrode structure.
These and other advantages of the present invention will become apparent to
those skilled in the art upon reading and understanding the following
detailed description with reference to the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view schematically showing an electrode
structure according to an example of the present invention.
FIG. 2 is an enlarged cross-sectional view showing an electrode layer in
the electrode structure according to an example of the present invention.
FIG. 3 is an enlarged cross-sectional view showing an interface portion
between an electrode layer and a semiconductor layer in the electrode
structure according to an example of the present invention.
FIG. 4 is a graph showing current-voltage characteristics of the respective
electrode structures of Example 1 and Comparative Example 1.
FIG. 5 is a graph showing the dependence of the current-voltage
characteristics of the electrode structure of Example 1 upon the annealing
temperature.
FIG. 6 is a graph showing current-voltage characteristics of the electrode
structures of Examples 3 and 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In this specification, a "nitride forming metal" refers to a metal whose
free energy is reduced when the metal is nitrified. In addition, the
expression "an electrode layer contains a mixture of a metal nitride and a
metal hydride" includes the following meanings: (1) the electrode layer
has a single-layered structure composed of the mixture; (2) the electrode
layer has a multi-layered structure including at least one layer
containing the metal nitride and at least one layer containing the metal
hydride; (3) the electrode layer includes a portion containing the metal
nitride and a portion containing the metal hydride; and (4) the electrode
layer includes a layer composed of a compound consisting of the metal
nitride and the metal hydride.
An exemplary electrode structure according to the present invention will be
described below with reference to FIGS. 1 to 3.
As shown in FIG. 1, the electrode structure of the invention includes: a
buffer layer 2; a semiconductor layer 3 and electrode layers 4 and 4' on a
substrate 1 in this order.
The substrate 1 can be made of sapphire, SiC or the like. The thickness of
the substrate 1 is preferably in the range of 10 to 500 .mu.m, and more
preferably in the range of 100 to 300 .mu.m.
The buffer layer 2 can be made of GaN, AlN or the like. The buffer layer 2
can be formed on the substrate 1 by a MOCVD method or the like. The
thickness of the buffer layer 2 is preferably in the range of 10 to 100
nm, and more preferably in the range of 10 to 50 nm.
The semiconductor layer 3 can be made of a p-type Al.sub.x Ga.sub.y
In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1)
semiconductor. The semiconductor layer 3 can be formed on the buffer layer
2 by a MOCVD method or the like. The thickness of the semiconductor layer
3 is preferably in the range of 2 to 6 .mu.m, and more preferably in the
range of 3 to 5 .mu.m. The conductivity type of the semiconductor layer 3
can be controlled by doping the semiconductor layer 3 with a p-type
impurity such as Mg.
Next, the electrode layers 4 and 4' will be described. Herein, for
simplification, only the electrode layer 4 will be described except for
the portion where the electrode layer 4' is required.
Examples of the metal nitrides contained in the electrode layer 4 include
ScN, TiN, VN, CrN, ZrN, NbN, LaN and TaN, which are produced from nitride
forming metals. Examples of the nitride forming metals include Sc, Ti, V,
Cr, Zr, Nb, La and Ta.
Examples of the metal hydrides contained in the electrode layer 4 include
ScH.sub.2, YH.sub.2, LaH.sub.2, CeH.sub.2, PrH.sub.2, NdH.sub.2,
SmH.sub.2, EuH.sub.2, YbH.sub.2, TiH.sub.2, ZrH.sub.2, HfH.sub.2, VH, NbH,
TaH and PdH, which are produced from hydrogen absorbing metals. Examples
of the hydrogen absorbing metals include Sc, Y, La, Ce, Pr, Nd, Sm, Eu,
Yb, Ti, Zr, Hf, V, Nb, Ta, Pd, Gd, Tb, Dy, Ho, Er and Tm.
In the previous paragraphs, Sc, Ti, V, Zr, Nb, La and Ta are cited as both
the nitride forming metals and the hydrogen absorbing metals. However, it
is noted that these metals are effective only as the nitride forming
metals, not as the hydrogen absorbing metals. The reason is as follows. In
the case where these metals are deposited on the semiconductor layer 3 to
form the electrode layer 4, these metals first react with nitrogen so as
to form the metal nitrides, but do not react with hydrogen. This is
because the number of nitrogen atoms existing in the semiconductor layer 3
is far larger than the number of hydrogen atoms bonded with the Mg atoms
in the semiconductor layer 3. Accordingly, hydrogen existing in the
semiconductor layer 3 is not attracted to the interface between the
semiconductor layer 3 and the electrode layer 4, so that the Mg atoms are
not activated. As a result, since the carrier density in the interface
between the semiconductor layer 3 and the electrode layer 4 becomes
insufficient, the formation of the electrode layer 4 on the semiconductor
layer 3 by depositing these metals on the semiconductor layer 3 does not
always improve the ohmic characteristics of the semiconductor device.
Consequently, it is preferable that Sc, Ti, V, Zr, Cr, Nb, La and Ta are
used as the nitrogen forming metals and Y, Ce, Pr, Nd, Sm, Eu, Yb, Hf Pd,
Gd, Tb, Dy, Ho, Er, and Tm are used as the hydrogen absorbing metals.
The thickness of the electrode layer 4 is preferably in the range of 100 to
500 nm, and more preferably in the range of 100 to 200 nm. The
cross-sectional shape of the electrode layer 4 may be an arbitrary shape,
e.g., circular, rectangular, polygonal, or the like. For example, in the
case where the electrode layer 4 has a circular cross section, i.e., in
the case where the electrode is cylindrical, the diameter of the cross
section is preferably in the range of 400 to 600 .mu.m, and the distance
between the centers of the two electrode layers 4 and 4' is preferably in
the range of 0.5 to 2 mm. However, these values are variable depending
upon the applications thereof.
The electrode layer 4 can be formed by an electron beam vapor deposition
method, a sputtering method or the like. For example, in the case where
the electrode layer 4 is formed by the electron beam vapor deposition
method, the ultimate background pressure is preferably 1.times.10.sup.-7
Torr or less, and the pressure during the deposition is preferably
5.times.10.sup.-7 Torr or less.
The electrode layer 4 can be formed by simultaneously depositing the
nitride forming metal and the hydrogen absorbing metal on the
semiconductor layer using the above-mentioned method, or the electrode
layer 4 can also be formed by depositing an intermetallic compound
containing the nitride forming metal and the hydrogen absorbing metal on
the semiconductor layer. Alternatively, the electrode layer 4 may be
formed by sequentially depositing the nitride forming metal and the
hydrogen absorbing metal in this order (or in an inverse order) on the
semiconductor layer, using the above-mentioned method. The electrode layer
4 may also be formed by alternately depositing the nitride forming metal
and the hydrogen absorbing metal several times on the semiconductor layer,
using the above-mentioned method.
FIG. 2 shows an exemplary state where a metal hydride layer 7 and a metal
nitride layer 8 are stacked in this order as the electrode layer 4 on the
semiconductor layer 3. Herein, GaN is used for the semiconductor layer 3;
Pd is used for the metal hydride layer 7; and Ti is used for the metal
nitride layer 8.
A chemical reaction proceeds at the interface between the electrode layer 4
consisting of the Pd layer 7 and the Ti layer 8 and the GaN layer 3, i.e.,
the interface between the Pd layer 7 and the GaN layer 3. This chemical
reaction can be caused when the Pd layer 7 and the Ti layer 8 are formed,
or when the annealing is performed after the Pd layer 7 and the Ti layer 8
are formed. That is to say, a metal hydride PdH can be generated by the
chemical reaction between Pd and hydrogen existing in the GaN layer 3, and
a metal nitride TiN can be generated by the chemical reaction between Ti
and nitrogen existing in the GaN layer 3. PdH and TiN thus generated can
exist in the interface between the electrode layer 4 and the GaN layer 3
in any of the following states: (1) A layer composed of the mixture of PdH
and TiN exists in the interface; (2) At least one layer containing PdH and
at least one layer containing TiN exist in the interface; (3) PdH exists
in at least a portion of the interface and TiN exists in at least a
portion of the interface; or (4) A layer composed of a compound of PdH and
TiN exists in the interface. The last state (4) is likely to be caused in
the case where Pd and Ti are simultaneously deposited on the semiconductor
layer 3.
In a preferred embodiment, after the electrode layer 4 is formed, an
annealing is performed. The annealing can be performed by an electric
furnace annealing method, a rapid thermal annealing (RTA) method or the
like. The annealing temperature is preferably in the range of 100.degree.
to 1000.degree. C., and more preferably in the range of 300.degree. to
500.degree. C. Although the annealing time is variable depending upon the
annealing temperature, the time is preferably in the range of 5 to 30
minutes, and more preferably in the range of 5 to 10 minutes. By
performing the annealing under these conditions, the reaction in the
interface between the semiconductor layer and the electrode layer is
promoted and the ohmic characteristics of the electrode structure are
further improved.
Next, referring to FIG. 3, the interface portion between the semiconductor
layer 3 and the electrode layer 4 will be briefly described below. FIG. 3
is an enlarged cross-sectional view showing the interface portion between
the semiconductor layer 3 and the electrode layer 4.
As shown in FIG. 3, a semiconductor layer 5 having a high carrier density
(a p.sup.+ -Al.sub.x Ga.sub.y In.sub.1-x-y N:Si (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor layer: hereinafter, this
layer will be called a "contact layer") is formed in the interface portion
between the semiconductor layer 3 and the electrode layer 4 in the
electrode structure thus obtained.
The carrier density of the contact layer 5 has become larger than the
carrier density of the semiconductor layer 3 before the electrode layer 4
is formed. That is to say, by forming the contact layer 5, it is possible
to realize an ideal ohmic contact showing an extremely small contact
resistance in the interface between the semiconductor layer 3 and the
electrode layer 4. Specifically, the carrier density of the contact layer
5 is preferably in the range of 10.sup.18 to 10.sup.20 cm.sup.-3, and more
preferably in the range of 10.sup.19 to 10.sup.20 cm.sup.-3.
According to the present invention, it is possible to realize an ideal
ohmic contact showing an extremely small contact resistance at the
interface between the semiconductor layer 3 and the electrode layer 4.
This effect can be obtained based on the following mechanism. The metal
hydride in the electrode layer 4 attracts hydrogen existing in the
semiconductor layer 3 to the interface between the semiconductor layer 3
and the electrode layer 4. As a result, the hydrogen atoms which have been
bonded with Mg are attracted to the metal hydride, so that fresh Mg atoms
are reproduced. In other words, the Mg atoms are activated. The activated
Mg atoms function as an effective acceptor impurity, thereby forming a
contact layer 5 having a high carrier density at the interface between the
semiconductor layer 3 and the electrode layer 4. The existence of the
contact layer 5 thus formed considerably reduces the width of the
potential barrier at the interface between the semiconductor layer 3 and
the electrode layer 4, thereby abruptly increasing the tunnel current
flowing through the interface. As a result, the contact resistance between
the semiconductor layer 3 and the electrode layer 4 becomes extremely
small, and therefore an ideal ohmic contact is realized.
In addition, according to the present invention, the metal nitride in the
electrode layer effectively attracts the nitrogen atoms existing in the
semiconductor layer 3 to the interface between the semiconductor layer 3
and the electrode layer 4. Accordingly, even if the nitrogen atoms
dissociate from the semiconductor layer 3 while the semiconductor layer 3
is being formed, so as to form the vacancies in the semiconductor layer 3,
especially in the vicinity of the surface of the semiconductor layer 3,
the nitrogen atoms attracted to the vicinity of the interface by the metal
nitride compensate for the vacancies by filling them. Consequently,
crystals having an appropriate stoichiometric ratio can be formed at the
interface between the semiconductor layer 3 and the electrode layer 4. As
a result, the portion in the vicinity of the surface of the semiconductor
layer 3 can be effectively turned into a p-type semiconductor.
Moreover, according to the present invention, the semiconductor layer 3 is
bonded with the electrode layer 4 at the interface therebetween because of
the reaction. Accordingly, an excellent adhesiveness is realized at the
interface between the semiconductor layer 3 and the electrode layer 4, so
that the electrode structure of the invention has excellent physical and
chemical properties. Such an electrode structure can be heat-treated at a
high temperature, e.g., about 1000.degree. C. Thus the annealing can be
performed more effectively in order to promote the reaction at the
interface and the activation of Mg, so that the ohmic characteristics are
further improved.
As described above, according to the present invention, since the metal
hydride and the metal nitride exist in the electrode layer, it is possible
to obtain an excellent electrode structure, which has excellent physical
and chemical properties, and an excellent ohmic contact for a p-type
semiconductor device.
Hereinafter, the present invention will be described by way of illustrative
examples with reference to the accompanying drawings. It is noted that the
present invention is not limited to the following specific examples.
Example 1
In the first example, an electrode structure as shown in FIG. 1 is
fabricated in the following way.
First, a buffer layer 2 made of GaN (thickness: 50 nm) is formed on a
sapphire substrate 1 by a MOCVD method. Next, a semiconductor layer 3 made
of an Mg-doped p-type GaN (thickness: 3 .mu.m) is formed on the buffer
layer 2 by a MOCVD method. The carrier density of the p-type GaN
semiconductor is 1.times.10.sup.18 cm.sup.-3. The electrode layers 4 and
4' are then formed on the semiconductor layer 3 by sequentially depositing
a metal hydride layer 7 made of Pd and a metal nitride layer 8 made of Ti,
in this order, on the semiconductor layer 3 inside a vacuum deposition
apparatus by an electron beam method. The thicknesses of the metal hydride
layer 7 and the metal nitride layer 8 are set to be 20 nm and 50 nm,
respectively. In forming the metal hydride layer 7 and the metal nitride
layer 8, the ultimate vacuum degree is 1.times.10.sup.-7 Torr, and the
vacuum degree at the deposition is 5.times.10.sup.-7 Torr. The electrode
layers 4 and 4' thus formed have a circular cross section having a
diameter of 500 .mu.m. The distance between the centers of these two
circular electrodes is 1 mm.
FIG. 4 shows the resulting current-voltage characteristics between the
electrode layers 4 and 4' of this electrode structure.
Comparative Example 1
In this comparative example, an electrode structure as shown in FIG. 1 is
fabricated in the same way as in the first example, except that Ni is used
instead of Pd and Ti as the electrode layers 4 and 4'. This electrode
structure corresponds to one of the electrode structures exhibiting
excellent ohmic characteristics disclosed in Japanese Laid-Open Patent
Publication No. 5-291621. FIG. 4 also shows the resulting current-voltage
characteristics between the electrode layers 4 and 4' of the electrode
structure of this comparative example, along with the results of the
above-described Example 1.
As is apparent from FIG. 4, the electrode structure of Example 1 exhibits
much more ideal ohmic characteristics as compared with those of the
electrode structure of Comparative Example 1.
Example 2
The effect of the annealing process on the electrode structure is inspected
in the following manner. The electrode structure obtained in Example 1 is
annealed in an electric furnace. The annealing process is performed three
times at temperatures of 100.degree. C., 300.degree. C. and 500.degree.
C., respectively. The annealing time is set to be 10 minutes in any of
these annealing processes. FIG. 5 shows the respective current-voltage
characteristics of the electrode structure annealed at the three
temperatures and those of the electrode structure of Example 1 which is
not annealed.
As is apparent from FIG. 5, the annealing process improves the ohmic
characteristics. The result shown in FIG. 5 also indicates that the higher
the annealing temperature is, the more ideal the ohmic characteristics
becomes, and that the ohmic characteristics are remarkably improved by
performing the annealing process at 300.degree. C. or higher. The results
shown in FIG. 5 also indicate that the electrode structure of the
invention has excellent physical and chemical properties. More
specifically, the crystal structure in the vicinity of the interfaces
between the semiconductor layer 3 and the electrode layers 4 and 4' in
this electrode structure can be annealed at 500.degree. C.
Example 3
In the third example, an electrode structure as shown in FIG. 1 is
fabricated in the same way as in the first example, except that Al.sub.0.3
Ga.sub.0.7 N having a carrier density of 5.times.10.sup.17 cm.sup.-3 is
used instead of GaN as the semiconductor layer 3, and that an
intermetallic compound containing Hf and Nb is used instead of Pd and Ti
as the electrode layers 4 and 4'. Accordingly, the electrode layers 4 and
4' of this electrode structure includes a metal hydride layer made of
HfH.sub.2 and a metal nitride layer made of NbN. FIG. 6 shows the
resulting current-voltage characteristics between the electrode layers 4
and 4' of the electrode structure of this example.
Example 4
In the fourth example, an electrode structure as shown in FIG. 1 is
fabricated in the same way as in the third example, except that In.sub.0.2
Ga.sub.0.8 N having a carrier density of 5.times.10.sup.17 cm.sup.-3 is
used instead of Al.sub.0.3 Ga.sub.0.7 N as the semiconductor layer 3. FIG.
6 also shows the resulting current-voltage characteristics between the
electrode layers 4 and 4' of the electrode structure of this example,
along with the results of Example 3.
As is apparent from FIG. 6, both the electrode structures obtained in
Examples 3 and 4 exhibit ideal ohmic characteristics. In particular, the
electrode structure obtained in Example 4 exhibits excellent ohmic
characteristics.
Example 5
In the fifth example, an electrode structure as shown in FIG. 1 is
fabricated in the same way as in the first example, except that various
combinations of the nitride forming metals and the hydrogen absorbing
metals shown in the following Table 1 are used as the electrode layers 4
and 4'. The resulting electrode structure is then annealed at 500.degree.
C. for ten minutes. The current-voltage characteristics between the
electrode layers 4 and 4' of the resulting electrode structure are
inspected so as to obtain the resistance values shown in Table 1.
TABLE 1
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Sc Ti V Cr Zr Nb La Ta
B (-61) (-74) (-35)
(-24) (-87)
(-51) (-65)
(-54)
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Y 29.3 20.2 62.5 70.8 15.8 40.5 28.7 36.7
Pd 15.0 13.9 35.6 50.3 10.0 17.2 14.5 16.3
Ce 29.7 28.1 74.0 98.7 20.3 34.0 30.8 35.2
Pr 43.8 35.6 55.3 60.2 30.5 38.3 44.3 37.2
Nd 49.7 38.6 60.3 70.6 31.8 40.7 48.3 39.3
Sm 15.1 14.2 36.0 48.3 11.0 18.3 15.9 17.2
Eu 22.6 21.3 65.3 72.7 20.7 35.3 30.5 35.2
Gd 20.4 15.8 58.7 60.3 15.6 30.3 24.6 25.2
Tb 13.7 14.0 38.3 48.1 11.3 18.1 15.7 17.3
Dy 47.2 45.3 87.1 87.3 24.3 48.3 47.1 49.2
Ho 37.6 18.8 52.7 55.3 17.6 41.7 40.3 42.6
Er 35.1 18.7 53.5 56.2 18.6 36.8 32.3 34.4
Tm 27.6 20.3 66.3 65.3 21.7 30.7 24.3 30.4
Yb 29.9 27.8 39.3 40.8 25.6 34.6 29.3 31.6
Hf 50.2 44.3 72.9 80.3 39.3 53.7 48.7 50.7
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(unit: .OMEGA.)
A: Nitride forming metal
B: Hydrogen absorbing metal
Note: The numerals in the parentheses shown under the respective nitride
forming metals denote the variation amount (kcal/mol) of the free energy
when the metals are nitrified.
As is apparent from Table 1, the electrode structure using the nitride
forming metal and the hydrogen absorbing metal shows low resistance
values, i.e., excellent ohmic characteristics.
As described above, according to the present invention, it is possible to
obtain an electrode structure having an ideal ohmic contact showing an
extremely small contact resistance between the semiconductor layer and the
electrode layer. In addition, crystals having an appropriate
stoichiometric ratio are formed in the interface between the semiconductor
layer and the electrode layer, so that an electrode structure having
excellent physical and chemical properties can be obtained.
By using such an electrode structure, a semiconductor device operating at a
lower operational voltage and exhibiting more ideal resistivity, e.g., a
blue-light-emitting diode, can be obtained as compared with the case of
using a conventional electrode structure.
Various other modifications will be apparent to and can be readily made by
those skilled in the art without departing from the scope and spirit of
this invention. Accordingly, it is not intended that the scope of the
claims appended hereto be limited to the description as set forth herein,
but rather that the claims be broadly construed.
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